The present invention relates generally to rechargeable batteries, and more particularly, to methods and apparatus for recharging batteries in the presence of a load current.
As electronic devices have become increasingly portable, the need for improved batteries has likewise increased. Rechargeable batteries are used in various electronic devices, such as portable audio devices, portable computers, cellular telephones and personal digital assistants (PDAs). A number of different types of rechargeable batteries exist, such as nickel cadmium (NiCd), nickel-metal hydride (NiMH), lithium ion (Li-ion) and lithium-polymer (Li-polymer) batteries.
The popularity of lithium-based rechargeable batteries has increased in recent years, primary due to improved energy densities, cycle life, output voltages and discharge rates, relative to nickel-based batteries. A number of techniques exist for charging lithium-based rechargeable batteries. Lithium-based rechargeable batteries include control circuitry that maintains the voltage and current of each cell within desired ranges during charge and discharge modes.
One conventional technique for charging lithium-based rechargeable batteries is referred to as constant-current/constant-voltage (CC/CV) charging. As shown in FIG. 1, a constant current is applied to a battery until a threshold voltage, VT, is reached, at a time tT. Typically, the threshold voltage for currently available lithium-based rechargeable batteries is 4.1 or 4.2 V per cell. Once the threshold voltage, VT, is reached, a constant voltage mode charges the battery with a constant voltage approximately equal to the threshold voltage, while the current gradually decreases to zero mA.
Another conventional technique for charging lithium-based rechargeable batteries is a pulse charging technique, shown in FIG. 2, where a constant current 210 is initially applied until the battery voltage exceeds the threshold voltage, VT. The current charge is then turned off until the battery voltage falls below the threshold voltage, VT. Thereafter, a series of current pulses 220 are applied. As shown in FIG. 2, the off time associated with each current pulse (the time required for the battery voltage to fall below the threshold voltage), such as the off times toff1 and toff2, progressively increases as charging progresses.
While these conventional techniques for charging lithium-based batteries perform effectively when there is no load present on the battery, they do not perform as effectively when a load is present on the battery. Typically, a heavy load may be present when a battery is recharging, such as when a cellular telephone is turned on during a recharge mode. The heavy load current required by the telephone causes inaccurate battery voltage measurements that can lead to overcharging and shortened battery life. The effect of the heavy load condition is that the battery voltage measured while the charge current is applied will be less than the threshold voltage, VT. Typically, conventional charging techniques tend to compensate by applying the charge current longer, and potentially overcharging the battery. A need therefore exists for an improved technique for measuring the battery voltage in the presence of a load current.
Generally, a method and apparatus are disclosed for recharging a battery in the presence of a load. The present invention utilizes the battery itself as an averaging element to maintain the battery at the desired threshold voltage, VT (on average) as the current charging source is selectively turned on and off. Generally, the present invention measures the battery voltage at the end of each interval just before the current source is turned on or off. A calculation is performed for each charging cycle (current on and off) to determine the duty cycle for the subsequent cycle. The calculated duty cycle is the percentage of time that the current source should be applied for the next cycle. The calculated duty cycle is based on the difference between the actual battery voltage and the desired voltage. In this manner, the on and off times are modulated by the present invention such that the average battery voltage, Vbat, is the desired threshold voltage, VT.
The present invention applies during the constant voltage mode when recharging a battery, such as a lithium-based battery or another battery employing CC/CV recharging techniques, in the presence of a load. Well-known techniques, such as the pulse charging techniques described above, may be applied for the constant current mode. The present invention requires only two measurements per charging cycle. Thus, the present invention demonstrates reduced power dissipation, relative to conventional techniques, due to fewer required analog-to-digital conversions.